Optics

Believe it or not the wave-particle duality goes way back 😉 Newton believed light was a stream of particles; he called them ‘corpuscles’. Christiaan Huygens who wasn’t as famous as old Sir Isaac, believed light to be a wave of some sort. It wasn’t until Thomas Young’sdouble slit experiment produced interference patterns from light that the wave nature of light was accepted.

The two slits in the double slit experiment emit light with a constant phase difference because part of each wavefront, from the point source, passes through each slit i.e. the light is coherent (has a constant phase difference). At any point where a bright fringe is formed the waves from the two slits arrive in phase (constructive interference). Where a dark fringe is formed, the waves are out of phase and cancel (destructive interference).

For two slits, S1 and S2, constructive interference occurs at S1R-S2R = nλ where n ∈ ℕ. n = 0 is the central bright fringe. λ/D = y/x, where D is the slit spacing, y is the fringe spacing and x is the screen distance.

The law of Reflection states that an incident beam is at the same angle as the reflected beam.

Images in a plane mirror are virtual images (can’t be projected) and are the same distance behind the plane as the object is in front of it.

Refraction: When passing from one medium to another, the angle of incidence i, and the angle of refraction r, obey the rule (Snell’s Law), sin i / sin r = constant, where the constant depends on the medium and is called the refractive index.

Light undergoes total internal reflection at a boundary between two transparent substances, provided the refractive index of the incident medium, ni > nr, the refractive index of the refracted medium. The angle of incidence is greater than the critical angle for the interface which is sin c = nr/ni

A real image is one that can be formed on a screen: Whereas a virtual image is formed where the light appears to be from.

Convex Lenses form real images by making the light converge to a point. A convex lens focal length is the distance from the lens to the focal point, which is where the beam, parallel to the lens axis, is brought to focus.

Concave lenses make light diverge. The focal length of a concave lens is the distance from the lens to the point where a beam of parallel light appears to emerge.

In optical instruments, the two main causes of image defect are; Spherical Aberration, where the outer rays are focused to a position different from the inner rays; and Chromatic Aberration, where the lens splits apart white light when it is refracted.